var num = Math.round(Math.random()*26) + 1; 
var s = '.gif';
if(num == 14 || num == 15) s='.jpg';
document.write('<a href="goto.cgi?a='+num+'" target="_blank"><img src="temp/in/'+num+s+'" border=0 alt="Реклама на Ru.Board"></a>');